In recent years, the consumption of natural gas, which is a clean fuel with less environmental pollution than petroleum or coal, has been rapidly increasing worldwide. Natural gas can be carried in a gaseous phase via onshore or offshore gas piping, or can be stored in a liquefied state in a liquefied gas carrier and transported to a remote source of demand.
For example, liquefied gas such as liquefied natural gas (LNG) or liquefied petroleum gas (LPG) is obtained by cooling natural gas or petroleum gas to a very low temperature (about −163° C., in the case of LNG) and is much more suitable for storage and transportation because the volume thereof is significantly lower than in a gaseous state.
Liquefied gas carriers such as LNG carriers are designed to carry liquefied gas to an onshore source of demand and, for this purpose, include a storage tank (commonly referred to as ‘cargo tank’) that is capable of withstanding ultra-low temperatures of the liquefied gas. In addition to LNG carriers, examples of an offshore structure provided with such a storage tank capable of storing liquefied gas under cryogenic conditions may include ships such as LNG RVs (regasification vessels) and plants such as LNG FSRUs (floating storage and regasification units), LNG FPSOs (floating production, storage and off-loadings), BMPPs (barge mounted power plant), and FSPPs (floating storage power plant).
An LNG RV is an LNG regasification facility installed on an LNG carrier capable of self-propulsion and floating. An LNG FSRU is a marine structure that stores LNG unloaded from an LNG carrier offshore at sea in a storage tank and regasifies LNG and supplies the regasified LNG to an onshore source of demand on demand, and an LNG FPSO is a marine structure that refines extracted natural gas at sea, liquefies the natural gas and stores the liquefied natural gas in a storage tank, and transfers the LNG from the storage tank to an LNG carrier on demand. A BMPP is a marine structure that produces electricity at sea by installing a power generation unit installed on a barge, and an FSPP is a marine plant that produces electricity by installing a power generation unit and a storage tank on a floating structure.
Such marine structures which transport or store a liquid cargo such as LNG, such as LNG carriers, LNG RVs, LNG FPSOs, LNG FSRUs, BMPPs, and FSPPs are equipped with a storage tank for storing LNG, which is a liquid cargo, under cryogenic conditions.
Such a storage tank is divided into an independent-type and a membrane-type depending on whether the weight of cargo is directly applied to an insulator. Typically, the membrane-type storage tank is divided into a GT NO 96-type and a TGZ Mark III-type, and the independent-type storage tank is divided into an MOSS-type and an IHI-SPB-type.
Referring to FIG. 1, a pump tower 20 is disposed inside a storage tank 10, particularly in the vicinity of a front wall 12 of the storage tank 10 to supply liquefied gas to the storage tank 10 or to discharge liquefied gas from the storage tank 10. The pump tower 20 is a large structure with a height of about 30 m for a typical liquefied gas carrier, although the height depends on the size of the liquefied gas storage tank 10.
A conventional pump tower 20 is formed in a substantially triangular column shape to withstand the flow of liquefied gas due to movement of a ship during sailing. Pipes 21, 22, 23 are disposed at respective corners of the triangular column to form a substantially triangular shape in plan view, and stiffeners 25 are interposed between the pipes 21, 22, 23 to connect and reinforce the pipes.
More specifically, as shown in FIG. 2, three pipes forming a substantially triangular shape include two discharge pipes 21, 22 each provided at a lower end thereof with a discharge pump (not shown) to discharge liquefied gas from the storage tank 10 and one emergency pipe 23 provided at a lower end thereof with an emergency pump (not shown). In addition to these three pipes, a filling pipe 24 for supplying liquefied gas into the storage tank 10 is further provided adjacent to the emergency pipe 23.
The pump tower 20 has an upper end suspended from the top of the storage tank 10 and is not directly secured to a bottom of the storage tank so as to cope with dimensional change due to thermal deformation of the storage tank 10. A lower end of the pump tower 20 is connected to a support disposed on a bottom of the storage tank 10 such that only the horizontal movement of the pump tower is restricted by the support, and vertical displacement of the pump tower is possible.
Since, due to shaking of a hull, flow of stored fluid, and the like, an external force is applied to the pump tower 20 in various directions, it is necessary to continuously study a support that can stably support the lower end of the pump tower, which is not secured.